The present invention relates to membranes for use in X-ray masks and, more particularly, thin films of B--Si--N system compounds whose residual stress is easily controlled and which exhibit a superior ability to transmit visible light employed for alignment and, further, to a method for preparing same.
In recent years, as transferring techniques for semiconductor integrated circuits which have become progressively finer, an exposure process using X-rays as a light source has been extensively studied or investigated.
X-rays masks used as original patterns in such a transferring process are made in such a basic structure that a desired pattern consisting of a material capable of preventing transmission of X-ray is formed onto a membrane (film) allowing transmission of X-ray which is tightly formed in an appropriate tension by a supporting frame arranged at the periphery of the membrane.
The membranes used in such X-ray masks are required to be superior in the following properties. (1) X-ray transmission, (2) flatness, (3) dimensional stability in plane, (4) mechanical strength, (5) transmission in the visible region, (6) chemical stability, etc. Among these properties, flatness and dimensional stability in plane are particularly significant in transferring with high precision and, for this purpose, the membrane should be placed in an appropriately strained condition by applying a low tension thereto. An excess tension leads to an increase in the distortion of the resulting X-ray mask and a reduction in the strength of the membrane. Compressive force is also unfavorable, since flexure problems arise. Further, in the exposure procedure, alignment is performed, for example, with the aid of diffraction of laser beam. Therefore, the membranes desirably have a high transmittance in the visible light region. Silicon nitride (hereinafter referred to as SiNx) and boron nitride (BNx) are greatly expected to be useful as membrane materials meeting the requirements set forth above and have been actually used for various studies.
The membranes made of those materials are usually formed onto an inorganic substrate such as silicon, etc., by thermal decomposition chemical vapor deposition (thermal CVD) or radio-frequency plasma chemical vapor deposition (RF plasma CVD) and then the central part of the substrate is removed by means of an etching process. The un-etched peripheral portion of the substrate is used as a supporting frame.
However, generally, in the BNx films prepared by CVD, compressive stress tends to remain and, in the SiNx films, a high tensile stress tends to remain. Therefore, attention is directed to control such tendencies.
Under such circumstances, there are currently single layered films of CVD-BNx or CVD-SiNx in which reduced residual tension has been actually realized and their stress has been controlled by intentionally preparing a composition having a deficiency in nitrogen relative to a stoichiometric composition (BN or Si.sub.3 N.sub.4). In this case, the optical absorption in the ultraviolet-visible region is increased due to an excess of silicon or boron. Consequently, transmittance in the visible region is inevitably, more or less, sacrificed in X-ray masks employing the membranes of these substances. Specifically, an example of the SiNx films is discussed by Sekimoto et al., in Journal of Vacuum Science and Technology, Vol. 21, page 1017 (1982) and an example of the BNx films is discussed by Dana and Maldonado, ibid., Vol. B4, page 235 (1986); or by Adams and Capio in Journal of Electrochemical Society, Vol. 127, page 399 (1980). Although the boron nitride films reported in the articles also contain hydrogen in a relatively large amount, the films are described in this specification as BNx films for simplification.
Alternatively, various attempts have been made in order to cancel out or control stress by stacking plural films having a different internal stress. For example, in Japanese Journal of Applied Physics, Vol. 20, page L669 (1981), Sekimoto et al reported a method of providing a trilayer structure of SiNx, SiO.sub.2 and SiNx. Further, in Journal of Vacuum Science and Technology, Vol. B4, page 221 (1986), Suzuki and Matsui describe that relaxation of the stress of a SiNx film can be effected by interposing an SiO.sub.2 film between the SiNx film and a silicon supporting frame, and thereby X-ray masks having a superior flatness can be obtained. However, needless to say, in such multilayered films, the production process thereof becomes complicated.
As further film formation processes, plasma CVD using electron cyclotron resonance (ECR) and sputtering are exemplified and most of those processes require certain extra procedures for controlling residual stress. Kiuchi et al reported in Extended Abstracts of the 44th Meeting of the Japan Society of Applied Physics (1983), page 236 that, in X-ray masks using Si.sub.3 N.sub.4 prepared by ECR plasma deposition process, the stress was controlled by thermal treatment after the film formation process. Further, at the same page of the Abstracts, Mochiji et al proposed a method for controlling stress by stacking a CVD-SiNx film on a BNx film formed by RF sputtering.
As a further method for controlling the residual stress, there may be mentioned the addition of a third element. For instance, with respect to the Si--O--N system, it has been reported by Rand and Roberts in Journal of Electrochemical Society, Vol. 120, page 446 (1973) and Csepregi and Heuberger in journal of Vacuum Science and Technology, Vol. 16, page 1962 (1979) that tensile stress can be reduced by increasing the oxygen content. However, such films are far inferior to nitride films in resistance to etching.
Further, U.S. Pat. Nos. 4 171 489 and 4 253 029 disclose processes for obtaining films having a low tension and an ability to transmit light by doping up to about 10 atomic % Si (U.S. Pat. No. 4 171 489) and 1-7 atomic % Si (U.S. Pat. No. 4 253 029) into BNx. Also, similar Si-doped BN films are described by Retajczyk, Jr and Sinha in Applied Physics Letters, Vol. 36, page 161 (1980).
BNx and SiNx films synthesized from the vapor phase are superior in the ability to transmit X-rays, mechanical strength and chemical stability and, thus, are suitable as membranes for use in X-rays masks.
However, in the X-ray masks using these nitride films, controlling their residual stress, which is caused during film formation and causes strain in the resulting films, has become an important problem.
For this, the currently used X-ray masks have the problems as set forth above. That is, the masks are inferior in transmission properties in the visible region or require multilayering process of two or more kinds of films or extra thermal treatment after film formation.